Microwave frequency control



L- E. NORTON MICROWAVE FREQUENCY CONTROL Filed Nov. 19, 1951 March 16, 1954 INVENTOR LDWELLE.NDHTEN ATTORNEY Patented Mar. 16 1954 MICROWAVE FREQUENCY CONTROL Lowell E. Norton, Princetom. N. J., assignor to Radio Corporation of America, a corporatio of Delaware Application November 19, 1951, Serial No. 257,109

1 Claim.

The present invention is related to microwave frequency control, and particularly to microwave frequency control with a gas cell.

It is known to employ a selectively resonant gas cell for the purposes of frequency control of an oscillator or generator of microwave energy. Examples of systems heretofore devised for the control or stabilization of microwave frequency generators may be found in the copending applications of William D. Hershberger, Serial No. 786,736, filed November 18, 1947; and Serial No. 4,497, filed January 27, 1948; and'of Lowell Norton, Serial No. 5,603, filed January 31, 1948 now U. S. Patent 2,559,730. These systems are satisfactory. However, they require considerable equipmentand are more or lesscomplex in structure and arrangement.

It is an object of the present invention to a microwave frequency stabilization. system' simplified over prior arrangements.

A further object of the invention is to provide an improved gas cell microwave frequency stabilization system, and one simplified in construction and arrangement.

In accordance with a preferred embodiment of the invention, a microwave generator is connected to supply energy to one arm of a first pair of arms of a bridge circuit, and through a molecular resonant gas cell to the other arm of the first pair. 'A detector is provided in each arm of the second pair. Energy directly from the generator and from the generator through the gas cell are thus applied to the detector arms from the bridge circuit. The detectors detect the energy thus applied to the input arms of the bridge circuit. Thedetectors are so connected that, in sense and magnitude, the low frequency output of the detectors combined corresponds in sense and magnitude primarily and critically to the differences in'phase of the signals applied to the first pair of arms. This combined output is applied to the generator for frequency control, preferably through a suitable filter and amplifier. The system of the invention takes advantage of the anomalous dispersion characteristic of the gas cell, to cause the phase variations with departure of generator frequency from the resonant. operating frequency of the gas cell. Therforefthev anomalous dispersion characteristic about the-resonant operating frequency of energy passed through the gas cell is employed to provide the simple and convenient arrangement of the invention.

Preferably the bridge circuit is a hollow pipe waveguide magic T. Also, preferably the lengths of lines (in terms of phase shift at the operating frequency) from the generator to the bridge plane of symmetry (or magic T junction) and from the generator through the gas cell to the bridge plane of symmetry differ by (2 a1)1k/2 radians of angular frequency or, what is the same, by an odd quarter wavelength at the resonant operating frequency. If the path difference of an odd quarter wavelength is thus accurately arranged, as by a line stretcher, one secures a greater degree of variation of control voltage with variations of the generator frequency from the gas resonance point'of the gas cell than otherwise. However, great exactness, although desirable, is not essential for a workable arrangement since the magnitude, of the control voltage decreases as cos 6, where 6 is the phase deviation from the desired 1r/2 difference. For small departure angles 6, cos 6 is very nearly unity and the decrease in control voltage is insignificant. 1

The foregoing and other objects, advantages, and novel features of the invention will be more apparent from the following description when taken in connection with the accompanying drawing in which:

Fig. 1 is a circuit diagram schematically'representing a preferred embodiment of the invention; and

Fig. 2 is a graph useful in explaining the .operation of th embodiment of Fig. 1.

Referring to Fig. l, a microwave generator l9 may be exemplified .by any suitable known oscillator, for example a klystron or magnetron, which has an element or means 12 for receiving a frequency control voltage. Various microwave generators of this kind are known, the frequency of oscillations being responsive to the voltage applied to the means I2.

An outputwaveguide I4 receives energy from the generator 10. From ajunction It of waveguides IB and 20 with waveguide 14, some of the generator energy from the generator output connection I4 is applied by waveguide it to a microwave resonant absorption gas cell 22.

The gas cell 22 may be of the type disclosed or employed in any of the above-identified codine app c io .The cr -Wave ner y transmitted through the" gas cell 22 "is applied by a waveguide 24 to one arm 30 of a magic T waveguide bridge 28. The waveguide 24 preferably includes means to vary the phase or path length of the microwave energy as indicated by a block 26 by suitable known means, generically termed a line stretcher.

The magic T 28 is one form of a bridge circuit. More generally, the bridge circuit employed in the invention has two pairs of arms and is characterized further by the fact that energy applied incident toward the bridge from one arm of one pair is e gually dividedbetween -.the,two larms of the other pair; gso jar as energy incident toward the bridge from any one arm of one pair is concerned, the other arm of the same pair is decoupled therefrom. Other bridge circuits are known which are so eharacterized,= forlexample i s so-called rat races, which aresuitable at the microwave frequencies here contemplated.

The arm 30 of magic T 28 receives the energy from the gas cell output waveguide 24. The arm 32 is paired with arni3fl and-receives the g0 microwave energy from the generator waveguide 20. The other pair of arms 34 and 36 have repeptilts hepnesitcl olari e cr a e to 38 and '40, 1pc ed equidistant from'thejuneti'on 28 and'the pl'ane of symmetry'be- 5 3 I,Preferably thearins 34 I fiectingor'matched terminaliphnnludgthe crystals of detectors '38 pectively Each of the crystal detectorsjid a'nd; All may include a b'y-pass' capacitor, 30 which maybe part ofthe crystal holders,or part y he output leads45= and iron j th crysta u/1th the walls'ofarms 34 and 36, any event,there should be sufii'cient' bypass"c'sit'p'ac'ity. to filter out the microwave frey, e filter ca pacities of'the detector s are; n gated schematically as "dotted g andfflgjor crystal detectors tlland at. i, sly con ected "b e i'o p leads; flfi 'and lll respectively and ground; The 40 detector output 182t1$ .5 '&lld148 are connected ionfillthrough acommoncdnput circuit of an amplifier 54. sister {ricpmprisi'ng saidlinputfcircuitmay y common,lqad'resistor for thedetectors38 & ,Zhe.a fhel fien 5 11s P erably a (direct current) amplifier 'which'at its output connection Sflsupplies the ,voltage fromc'onnectiongs l', amplifi audwlth' s'ensingito the'freu s yi .QQ, Q.1.}.l &QLZ L2 f 11 generator Howeyer the sensing may be about a particular voltage level which continuously .biases th'e frequeasy control means vThe characteristics* of t the amplifier g5}; depend on the generator .I 0 and the type or frequency control means 12 whichis employe d and in fact, the amplifier 54" maybe considered a partoj the f requency control means rig-spam "on s up sethe generator l0 tohave an outputonconuection l4 of,a frequency' f, and withl aisofallcdflll ular frequency e=21wfi The frequency I should be and is assumed to be very near the resonant jrequencyf -oilihe gas irr'gas r cell 23; Star withinl 9. .1. an w very. n z t z'w' ft- The energy passing through the gas cell 22*may be considered as passing through a medium hav ing an'effective index of retraction n; where" where N=nuiiilier of molecules per; unit volume; 75

4 m and e are respectively the mass and charge of the molecule as an oscillator in the electromagnetic field of the microwave energy in gas cell; and 1- is the mean time interval between molecular collisions.

The above formula is derived in the article by Van Vleck and Weisskopf, volume 17 of the Review of Modern Physics (1945), starting at page 227, entitled On the Shape of Collision-Broadened Lines, and may be found at page 232, Formula 18 st the articl. I

Now 11 -1 is very much less than unity,- so that the expression on the right hand side of Formula 1 above is very small, say less than a tenth, and in practical-cases less than 1%. Therefore, to a good approximation one way write:

(2 t. r 2 21rne.

Let Aw; 1/1, so that An: corresponds substantially to the expected energy spectrum line'width or band width due to collision broadening of molecules in the gas. Thenfrom (2), as an identity, may be derived the following;

If v is the phase velocity of electromagnetic waves of energypassing through the gas medium:

6 (4) fifth.

where'k isthe wavelength'in' the medium. There-- fore the change in phase in propagation through a path length L of the mediumis whereswg, is the free space wavelength of microwave energy of frequency ft.

The expression for-'n'aiforded by Equation 3 may be substituted in Equation 5 to yield:

if the power out of the gas cell") to waveguide 24 (7) P=(1 -M$- P'r at the frequency f =,f oit a.- Forfrequencies no equal to or differin from w by less than a few per cent, it may be shown that A t- I approximately. Equation 8 above maybe derived from consideration of the absorption coefli cient a formula forjiwhich may be found at page 230 of the said Yan Vleck and Weisslgopi papenFIIr sewer cu s m owayecas'eyth'e app. "s. proximation of (8) is very nearanexact value being much closer than 1%.

.Now the. field strength at one detector due to energy F1 from arm 32 may be written as E1= P1 sin rut and that at the same detector due to Be (10) E2= WEI? assuming that the connections are so arranged that the total phase difference at the frequency oi=w from junction I6 to the plane of symmetry between arms 30 and 32 by the two energy paths is (2a1)1r/2 radians or an odd quarter wavelength at the mean operating frequency in, plus the phase shift through the gas cell. Then the angle is the frequency dependent phase angle A and that from theother the amplitude of V2 where The detector outputs are combined in opposition, so that the net output voltage has the amplitude of V where (13) V=4K1E1Ez The voltage V appears at junction 50. Therefore,

36 2K K P 1-- [sin(2wt+)-sin] But the by-pass capacities 42 and. 44 operate to filter out the microwave frequency of (14). Therefore the voltage at junction 50 is substituting from (6),

M Le:

From IE it is apparent that, for w=w if the phase shift s through the gas cell I0 is zero, by suitable choice of L and the difference between the waveguide lengths of waveguides 20 and 24 from junction 16 to the symmetry plane between arms and 32 is a quarter wavelength (as postulated by Equations 9 and 10) then there is zero voltages at the junction 50. As the angular frequency w changes, causingsinp to-change, from =0, the voltage V changes also. Furthermore,as increases-or decreases from'the value =0 (or 1r) with increasing or decreasing w, the sign of V changes. Therefore, V hassensing according to the-departure of (,5 from zero and with'the departure of w from the value to make =0. But this value of w is very near n In fact, at microwave frequencies of about 24x10 .(corresponding to an ammonia gas spectral lineof an ammonia gas cell often employed for stabilization) the error in accepting =0 as the true value for when w=w,,, would be extremely small.

If the phase shift through the gas cell [0 is not zero, it is clearthat suitable adjustment of the line stretcher 26 of waveguide 24 will cause the voltage at the plane of symmetry of arms 30 and 32 to be an odd quarter wavelength out of phase at the operating frequency w=w,,. Therefore it is clear that regardless of the length L, suitable adjustment of the adjustable phase or path length portion 26 of line 24 will make the device operable in the manner described.

An explanationless quantitative in nature of the operation of the arrangement of Fig. 1 may quency in the near neighborhood of a resonance:

frequency in for energy passed through the gas cell H). To a close approximation, 'n1.=0 for microwave frequency .f=fo. This curve may be thought of as a plot ofthe 11-1 of Equation 3 above. This curve may also be considered as a. phase shift curve. The portion above the. axis represents phase shift in one sense, and that be-- low the axis represents phase shift in areverser or opposite sense. The reference voltage may be: thought of as the microwave voltage output of. the gas cell 22 at the frequency in. Then, the energy from waveguide 24 due to the gas cell. 22, advances or retards (or vice versa) in phasein the fashion shown by the graph of Fig. 2,. with frequency above or below in.

Now suppose the line length or phase of the adjustable path length 26 to be so adjusted that with the generator [0 generating oscillations of frequency f=f0, and at the plane of symmetry between arms 30 and 32, the energy from the arms 30 and 32 arrives 1r/2 radians out of phase with each other. Now energy from arm- 30 is in phase at equal distances in arms 34 and 36 from the junction; and energy from arm 32 is in opposite phase at equal distances in arms 34 and 36 from the junction. Therefore in one of arms 34 and 36 energy from arm 30 leads that from arm 32 by 1r/2 radians; and in the other arm of arms 34 and 36, energy from arm 30 lags that from arm 32 by'1r/2 radians. Therefore the fields at the detectors have the same amplitudes. If the detectors are appropriately connected, the voltages developed by the detectors and responsive each to the field in its respective arms, cancel or Oppose at the junction 50.

Now if the frequency f from the generator varies from it, the phase of the microwave energy at the magic T 28 from arm 30 is advanced or retarded, substantially according to the graph of Fig. 2, relative to the microwave energy from arm 32, which is correspondingly retarded or advanced. Therefore, depending on whether in one 

